1. Field of the Invention
The present invention relates to a surface light source capable of changing a range of a spread angle of exit light, and a liquid crystal display apparatus that can vary a viewing angle by using this surface light source.
2. Description of the Related Art
A liquid crystal display apparatus has a structure in which a surface light source that emits light having a substantially uniform intensity to a substantially entire surface of a liquid crystal display device is arranged on an opposite side of an observation side of the liquid crystal display device having a screen region in which a plurality of pixels that control transmission of light are aligned in a matrix form. This surface light source is constituted of a light guide plate and light-emitting elements. The light guide plate is formed of a plate-like transparent member having a structure in which an incidence facet from which light enters is formed on at least one facet of this transparent member, an exit surface from which light that has entered from the incidence facet exits is formed on one of two plate surfaces, and a reflection surface on which light that has entered from the incidence facet is reflected toward the exit surface is formed on the other plate surface. The light-emitting elements are arranged to face the incidence facet of the light guide plate, and emit light toward the incidence facet (Jpn. Pat. Appln. KOKAI Publication No. 2000-113706).
Generally, the liquid crystal display apparatus is designed to obtain a wide viewing angle, and hence a third person may possibly peep display of this apparatus from a direction inclined toward a front direction (a normal line direction of the liquid crystal display device).
Therefore, there has been proposed a liquid crystal display apparatus that can perform wide viewing angle display and narrow viewing angle display with high security properties that is not peeped by a third person by arranging a viewing angle restricting liquid crystal element on one surface side of the liquid crystal display device to lower visibility from a direction inclined with respect to a front direction (Jpn. Pat. Appln. KOKAI Publication No. 2004-133334).
According to this viewing angle restricting liquid crystal element, electrodes having predetermined shapes are provided on opposed inner surfaces of a pair of substrates in accordance with each sectional region, and liquid crystal molecules in a liquid crystal layer provided between the pair of substrates are aligned in an alignment state having a viewing angle in one direction with respect to a normal line direction of the liquid crystal display device and another alignment state having a viewing angle in a direction inclined in an opposite direction in accordance with each of the sectional regions.
This liquid crystal display apparatus applies a voltage to a space between electrodes of the viewing angle restricting liquid crystal element to lower visibility from an oblique direction, thereby restricting a viewing angle of a display image in the liquid crystal display device.
That is, when a voltage is not applied to the space between the electrodes of the viewing angle restricting liquid crystal element, the viewing angle restricting liquid crystal element enters a non-display state, and a display image of the liquid crystal display device can be seen with a wide viewing angle. On the other hand, when a voltage is applied to the space between the electrodes of the viewing angle restricting liquid crystal element, a display image of the liquid crystal display device is hidden by display of the viewing angle restricting liquid crystal element as seen from a direction inclined toward one side with respect to the front direction and a direction inclined toward the other side. That is, the display image of the liquid crystal display device is hidden by display in each sectional region having a viewing angle in the direction inclined toward one side of the viewing angle restricting liquid crystal element and display corresponding to the electrodes having the predetermined shapes in the other sectional region having a viewing angle in the direction inclined toward the opposite side. Therefore, the display image of the liquid crystal display device cannot be recognized from the one direction and the direction inclined to the opposite side, the viewing angle of the display image is apparently restricted, and the viewing angle of the display image is narrowed.
However, according the viewing angle restricting liquid crystal element in which liquid crystal molecules are aligned in different alignment states in accordance with each sectional region obtained by dividing a region corresponding to a screen of the liquid crystal display device, complicated alignment processing (rubbing processing of an alignment film) whose direction varies in accordance with each sectional region must be applied to each of the inner surfaces of the pair of substrates, and hence manufacture is difficult.